Regulation of the human platelet contractile system is effected through the actin-filament, similarly to vertebrate muscles and not through myosin as in molluscan muscles. Pure rabbit muscle actin, devoid of tropomyosin and troponin, activates the Mg-ATPase activity of platelet natural actomyosin (thrombosthenin) in the absence of Ca ions. Furthermore, thin filaments prepared by polymerization of crude platelet G-actin and containing tropomyosin and probably troponin inhibit the actin-activated Mg-ATPase activity of rabbit myosin, in the absence of Ca ions. These results indicate that the platelet contractile regulation is of the "actin-linked" type and is mediated by tropomyosin and troponin.